sfxge: remove obsolete common code PKTFILTER module

The pktfilter module has been obsolete for some time, as
it was replaced by newer features in filter module. With
the removal of the storport driver, this module has no
users and can be removed.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4875